In recent years, a display device such as a liquid crystal display used for map display in a car navigation system includes an optical sensor for adjusting brightness of a backlight in the display. The brightness of the backlight is controlled to be dimmer when the vehicle is in a dark place, or to be brighter when the vehicle is in a very well-lighted place. In this manner, glaring or darkness of map display in the liquid crystal display is prevented. Japanese patent document JP-A-H11-316566 discloses a liquid crystal display having above-described control of brightness.
The optical sensor such as a photo diode or the like has an output characteristic that is susceptible to temperature of an environment. That is, the optical sensor receiving the same brightness outputs a different amount of output current when the temperature of the environment is different. As a result, the brightness of the liquid crystal display may be too bright or too dim for a driver of the vehicle when the output current from the optical sensor is changed because of the temperature of the environment.
Similar problems are experienced when the optical sensor is used in other kinds of devices. For example, a cellular phone having brightness detecting function for controlling volume of a ring tone does not work properly when the optical sensor does not distinguish the brightness in a pocket from the brightness in a bag because of the output characteristic under the influence of the temperature. That is, the volume of the ring tone does not change for drawing attention of a user even when the cellular phone is stored in the bag.